Molar mass
Type a chemical formula: get the molar mass in g/mol and each element's contribution, with the percent composition.
Examples: H2O · NaCl · Ca(OH)2 · CuSO4·5H2O · Al2(SO4)3
What it's for
Molar mass (grams per mole) is the starting point of every stoichiometry exercise: from moles to grams and back. The tool handles nested parentheses, coefficients and hydrated salts.
Mind the case
Every element starts with a capital letter: «CO» is carbon + oxygen, «Co» is cobalt. It's the most common mistake when a formula «doesn't add up».
What a mole is, in two lines
A mole is simply a number of pieces, like a dozen: 6.022×10²³ atoms or molecules. The reason it's that number and not a round one is practical: it's chosen so that the mass of one mole, in grams, is the same number as the atomic mass read off the periodic table. Carbon has atomic mass 12, and a mole of carbon weighs 12 grams. That's all there is to it, and it's what makes chemistry calculable with kitchen scales instead of by counting atoms.
How to read the result
The molar mass (in grams per mole) is the sum of the masses of every atom in the formula. From there come the two sums always needed: from moles to grams you multiply, from grams to moles you divide. For water, at 18 g/mol, half a mole is 9 grams and 45 grams is two and a half moles.
The percentage composition answers a different question, one that comes up constantly in problems: how much of the weight is due to a given element. In water, oxygen is 89% of the weight while being one atom in three: it's the quickest way to see why a formula «weighs» what it weighs.
The shapes a formula can take
It reads the nested brackets, so Ca(OH)₂ and Al₂(SO₄)₃ can be written as they are; the coefficients in front of the formula, so «2 H2O» counts double; and hydrated salts with the dot, like CuSO4·5H2O, where the water of crystallization is part of the weight and has to be counted. It's the part hand calculations get wrong most often, because it's easy to forget that the subscript outside a bracket multiplies everything inside it.
Nearby tools
To balance a reaction, with the atoms counted on both sides, there is Balance a chemical equation. The symbols and atomic numbers are in Periodic table, unit multiples in SI prefixes and the Greek letters of formulas in Greek alphabet.